Exhaust system having doc regeneration strategy

ABSTRACT

An exhaust system for use with a combustion engine is disclosed. The exhaust system may have an exhaust passage configured to receive a flow of exhaust from the combustion engine, and an oxidation catalyst disposed within the exhaust passage. The exhaust system may also have a fuel injector configured to selectively inject fuel into the exhaust at a location upstream of the oxidation catalyst, a temperature sensor configured to generate a signal indicative of a temperature of exhaust flowing through the exhaust passage, and a controller in communication with the fuel injector and the temperature sensor. The controller may be configured to make a determination based on the signal that an oxide layer has formed on the oxidation catalyst, and to regulate operation of the fuel injector to inject fuel and reduce the oxide layer based on the determination.

TECHNICAL FIELD

The present disclosure is directed to an exhaust system and, more particularly, to an exhaust system having a strategy for regenerating a diesel oxidation catalyst (DOC).

BACKGROUND

Diesel oxidation catalysts (DOCs) are commonly used in the exhaust systems of internal combustion engines to facilitate different emission reduction processes. For example, DOCs can be used to create a desired ratio of NO to NO₂ in an engine's exhaust stream that enhances NO_(X) reduction within a downstream selective catalytic reduction (SCR) device. In another example, DOCs can be used to increase an overall amount of NO₂ in the exhaust stream passing through a diesel particulate filter (DPF) to lower a combustion temperature of particulate matter trapped in the DPF and thereby enhance passive regeneration of the DPF.

Although effective as exhaust treatment tools, DOCs can also be problematic under some conditions. That is, it has been found that the active catalytic materials of a DOC, which commonly include precious materials such as Platinum, can become less active when exposed to high temperatures. Consequently, as the DOC cools after reaching high temperatures or operates for an extended period of time at the high temperatures, the DOC is less capable of generating NO₂.

The reduced functionality of a DOC after exposure to high temperatures mentioned above is discussed in a journal article titled “Inverse Hysteresis During The NO Oxidation on Pt Under Lean Conditions,” written by W. Hauptmann et al. and published on Sep. 16, 2009 (“the Hauptmann article”). In the Hauptmann article, a method of regenerating a DOC is also discussed. In particular, the Hauptmann article describes how cooling the DOC slowly over an extended period of time in an exhaust flow containing a high concentration of NO can improve subsequent performance of the DOC.

SUMMARY

One aspect of the present disclosure is directed to an exhaust system for use with a combustion engine. The exhaust system may include an exhaust passage configured to receive a flow of exhaust from the combustion engine, and an oxidation catalyst disposed within the exhaust passage. The exhaust system may also include a fuel injector configured to selectively inject fuel into the exhaust at a location upstream of the oxidation catalyst, a temperature sensor configured to generate a signal indicative of a temperature of exhaust flowing through the exhaust passage, and a controller in communication with the fuel injector and the temperature sensor. The controller may be configured to make a determination based on the signal that an oxide layer has formed on the oxidation catalyst, and to regulate operation of the fuel injector to inject fuel and reduce the oxide layer based on the determination.

Another aspect of the present disclosure is directed to a method of operating an exhaust system. The method may include directing exhaust through an oxidation catalyst, and making a determination that an oxide layer has formed on the oxidation catalyst. The method may further include selectively introducing a burst of fuel into exhaust directed through the oxidation catalyst to reduce the oxide layer based on the determination.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic and diagrammatic illustration of an exemplary disclosed power system; and

FIG. 2 is a graph illustrating an operation of the power system of FIG. 1.

DETAILED DESCRIPTION

FIG. 1 illustrates an exemplary power system 10. For the purposes of this disclosure, power system 10 is depicted and described as a diesel-fueled internal combustion engine. However, it is contemplated that power system 10 may embody any other type of combustion engine such as, for example, a gasoline engine or a gaseous fuel-powered engine burning compressed or liquefied natural gas, propane, or methane. Power system 10 may include an engine block 12 that at least partially defines a plurality of combustion chambers 14 provided with fuel via a plurality of fuel injectors 15. It is contemplated that power system 10 may include any number of combustion chambers 14 and that combustion chambers 14 may be disposed in an “in-line” configuration, a “V” configuration, or in any other conventional configuration.

Multiple separate sub-system may be included within power system 10. For example, power system 10 may include an air induction system 16, an exhaust system 18, and a control system 20. Air induction system 16 may be configured to direct air into combustion chambers 14 of power system 10 to mix with fuel from injectors 15 for subsequent combustion. Exhaust system 18 may exhaust byproducts of the combustion to the atmosphere. Control system 20 may regulate operations of air induction and exhaust systems 16, 18 to reduce the production of regulated constituents and/or their discharge to the atmosphere.

Air induction system 16 may include multiple components that cooperate to condition and introduce compressed air into combustion chambers 14. For example, air induction system 16 may include an air cooler 22 located downstream of one or more compressors 24. Compressors 24 may be connected to pressurize inlet air directed through cooler 22. A throttle valve (not shown) may be located upstream and/or downstream of compressors 24 to selectively regulate (i.e., restrict) the flow of inlet air into power system 10. A restriction on the flow of inlet air may result in less air entering power system 10 and, thus, affect an air-to-fuel ratio of power system 10. It is contemplated that air induction system 16 may include different or additional components than described above such as, for example, variable valve actuators associated with each combustion chamber 14, filtering components, compressor bypass components, and other known components that may be selectively controlled to affect the air-to-fuel ratio of power system 10, if desired. It is further contemplated that compressors 24 and/or cooler 22 may be omitted, if a naturally aspirated power system 10 is desired.

Exhaust system 18 may include multiple components that condition and direct exhaust from combustion chambers 14 to the atmosphere. For example, exhaust system 18 may include an exhaust passage 26, one or more turbines 28 driven by exhaust flowing through passage 26, and a plurality of exhaust treatment devices fluidly connected within passage 26 at a location downstream of turbines 28. It is contemplated that exhaust system 18 may include different or additional components than described above such as, for example, exhaust gas recirculation (EGR) components, bypass components, an exhaust compression or restriction brake, an attenuation device, and other known components, if desired.

Each turbine 28 may be located to receive exhaust discharged from combustion chambers 14, and may be connected to one or more compressors 24 of air induction system 16 by way of a common shaft 30 to form a turbocharger. As the hot exhaust gases exiting power system 10 move through turbine 28 and expand against vanes (not shown) thereof, turbine 28 may rotate and drive the connected compressor 24 to pressurize inlet air. In one embodiment, turbine 28 may be a variable geometry turbine (VGT) or include a combination of variable and fixed geometry turbines. VGTs are a type of turbocharger having geometry adjustable to attain different aspect ratios, such that adequate boost pressure may be supplied to combustion chambers 14 under a range of operational conditions. As a flow area of turbine 28 changes, the air-to-fuel ratio and thus the performance of power system 10 may also change. Alternatively, a fixed geometry turbocharger with or without an electronically controlled wastegate may be included, if desired.

The treatment devices of exhaust system 18 may receive exhaust from turbine 28 and reduce or remove constituents of the exhaust. In one example, the exhaust treatment devices may include one or more of a diesel particulate filter (DPF) 32 and a selective catalytic reduction (SCR) device 34. A particulate filter is a device designed to trap particulate matter and typically consists of a wire mesh or ceramic honeycomb medium. As exhaust laden with particulate matter passes through the filter, the particulate matter is blocked by the filter and suspended from the exhaust flow. SCR device 34 may include a catalyst substrate 36 located downstream from an injector 38. A pressurized gaseous or liquid reductant, most commonly urea (NH₂)₂CO or a water/urea mixture may be selectively advanced into the exhaust upstream of catalyst substrate 36 by injector 38. An onboard reductant supply 40 and a pressurizing device 42 may be associated with injector 38 to provide the pressurized reductant. As the injected reductant is adsorbed onto a surface of catalyst substrate 36, the reductant may react with NOx (NO and NO₂) in the exhaust gas to form water (H₂O) and diatomic nitrogen (N₂).

The performance of particulate trap 32 and/or SCR device 34 may be enhanced by an upstream-located diesel oxidation catalyst (DOC) 44. In particular, as DPF 32 operates, particulate matter may build up therein and, if unaccounted for, eventually restrict the exhaust flow through DPF 32 by an undesired amount. Accordingly, DPF 32 may be selectively regenerated to reduce the amount of particulate matter buildup. To initiate regeneration of DPF 32, the temperature of the particulate matter entrained within DPF 32 must be elevated above a combustion threshold temperature at which the trapped particulate matter is burned away, for example above about 600° C. Under most conditions, however, this threshold temperature is not achieved naturally. DOC 44, as will be explained in more detail below, may generate an amount of NO₂ in the exhaust flow passing through DPF 32 that helps to lower the combustion threshold temperature to a level that allows combustion of trapped particulate matter under normal operating conditions. This type of regeneration may be known as passive regeneration. Similarly, the reduction process performed by SCR device 34 may be most effective when a concentration of NO to NO₂ supplied to SCR device is about 1:1, and DOC 44 may help provide this concentration.

DOC 44 may include a porous ceramic honeycomb structure or a metal mesh substrate coated with a material, for example a washcoat containing precious metals, that catalyzes a chemical reaction to alter the composition of the exhaust. For example, DOC 44 may include a washcoat of palladium, platinum, vanadium, or a mixture thereof that facilitates the conversion of a portion of the NO already existing in the exhaust flow of power system 10 to NO₂. The exhaust flow having an increased amount of NO₂ may then be directed into DPF 32 to facilitate passive regeneration therein and/or into SCR device 34 to facilitate the reduction of NO_(X).

As described above, the precious metal/active catalytic component of a DOC can become less active after exposure to high temperatures. In particular, it has been determined that when exposed to high temperatures for an extended period of time, a DOC can become coated with an oxide layer due to the NO₂ it generates, the oxide layer resulting in decreased performance of the DOC. This phenomenon is depicted in FIG. 2. Specifically, FIG. 2 illustrates a first curve 200 representing operation of a typical DOC during an initial operation of a corresponding power source as the power source heats up, and a second curve 210 representing operation of the same DOC after an extended period of time at elevated temperatures and during cooling of the power source. As can be seen from comparison of these curves, the typical DOC will initially perform well and convert an increasing amount of NO to NO₂ until exhaust temperatures reach about 180° C. Once exhaust temperatures reach about 180° C. and continue to increase, however, the conversion efficiency of the typical DOC may reduce. And, the conversion efficiency drops dramatically as the DOC cools after extended operation at elevated temperatures. For example, when cooling from about 250° C. to about 200° C., the efficiency shown in curve 210 may be about half of the initial efficiency shown in curve 200. Further, if reheated before sufficient cooling has occurred, the efficiency of the typical DOC will follow the reduced efficiency curve 210 rather than curve 200.

To help ensure prolonged operation of DOC 44 at a desired level, an injector 46 may be disposed at a location upstream of DOC 44 and configured to selectively inject bursts of a hydrocarbon, for example diesel fuel, into exhaust passage 26. When the hydrocarbon comes into contact with the oxide layer on the metallic substrate of DOC 44, a chemical reaction may occur that removes or otherwise reduces the oxide layer. An onboard hydrocarbon supply 48 and a pressurizing device 50 may be associated with injector 46 to provide the pressurized hydrocarbon.

Control system 20 may include components configured to regulate the treatment of exhaust from power system 10 prior to discharge to the atmosphere. Specifically, control system 20 may include a controller 52 in communication with one or more exhaust sensors 54, injector 38, and injector 46. Based on input from exhaust sensor 54 and/or other input, controller 52 may determine an amount of NO_(X) being produced by power system 10, a performance of SCR device 34, the formation of the oxide layer on DOC 44, a desired amount of urea that should be sprayed by injector 38 into the exhaust flow, a desired amount of hydrocarbon that should be sprayed by injector 46 into the exhaust flow, and/or other similar control parameters. Controller 52 may then regulate operation of injectors 38 and 46 such that the desired amounts of urea and hydrocarbon are sprayed into the exhaust flow upstream of catalyst substrate 36 and DOC 44, respectively.

Controller 52 may embody a single or multiple microprocessors, field programmable gate arrays (FPGAs), digital signal processors (DSPs), etc. that include a means for controlling an operation of power system 10 in response to signals received from the various sensors. Numerous commercially available microprocessors can be configured to perform the functions of controller 52. It should be appreciated that controller 52 could readily embody a microprocessor separate from that controlling other non-exhaust related power system functions, or that controller 52 could be integral with a general power system microprocessor and be capable of controlling numerous power system functions and modes of operation. If separate from the general power system microprocessor, controller 52 may communicate with the general power system microprocessor via datalinks or other methods. Various other known circuits may be associated with controller 52, including power supply circuitry, signal-conditioning circuitry, actuator driver circuitry (i.e., circuitry powering solenoids, motors, or piezo actuators), communication circuitry, and other appropriate circuitry.

Exhaust sensor 54 of control system 20 may be configured to generate a signal indicative of formation of the oxide layer on the metallic substrate of DOC 44. In one embodiment, exhaust sensor 54 may be a temperature sensor configured to generate a signal corresponding to a temperature of the exhaust passing through DOC 44, and send this signal to controller 52. In this example, when the signal corresponds with an exhaust temperature above a threshold temperature, controller 52 may make a determination that it is likely that the oxide layer has formed. The threshold temperature may be about 180-250° C. and correspond with about 60-100% of the substrate of DOC 44 being covered by the oxide layer. In another example, when the signal indicates that the exhaust temperature has remained above the threshold temperature for a threshold period of time, controller 52 may make the determination that the oxide layer has formed. The threshold period of time may be about 2-30 seconds. It is contemplated that exhaust sensor 54 may be a type of sensor other than a temperature sensor, if desired, and that controller 52 may be configured to similarly make the determination regarding formation of the oxide layer based on the corresponding signal(s) from that sensor. For example, exhaust sensor 54 could alternatively embody a NO_(X) sensor configured to detect an amount of NO and/or NO₂ in the exhaust exiting DOC 44, with controller 52 then being configured to determine formation of the oxide layer based on the sensed conversion performance of DOC 44. In another example, exhaust sensor 54 could alternatively embody a particulate filter sensor configured to generate signals indicative of lower than expected regeneration rates of DPF 32 as determined by an amount of soot detected within or downstream of DPF 32, and/or a pressure differential across DPF 32.

It is further contemplated that sensor 54 may alternatively embody a virtual sensor. A virtual sensor may produce a model-driven estimate based on one or more known or sensed operational parameters of power system 10 and/or DOC 44. For example, based on a known operating speed, load, temperature, boost pressure, ambient conditions (humidity, pressure, temperature), and/or other parameter of power system 10, a model may be referenced to determine formation of the oxide layer on DOC 44. Similarly, based on a known or estimated NOx production of power system 10, a flow rate of exhaust exiting power system 10, and/or a temperature of the exhaust, the model may be referenced to determine the formation of the oxide layer. As a result, the signal directed from sensor 54 to controller 52 may be based on calculated and/or estimated values rather than direct measurements, if desired. It is contemplated that rather than a separate element, these virtual sensing functions may be accomplished by controller 52, if desired.

When controller 52 determines that it is likely that the oxide layer has formed on the substrate of DOC 44, controller 52 may selectively cause injector 46 to inject one or more bursts of hydrocarbon into the exhaust at a location upstream of DOC 44. For example, after the temperature of the exhaust at DOC 44 exceeds 180° C. and/or remains above 180° C. for at least 2 seconds, controller 52 may energize injector 46 to inject an amount of hydrocarbon necessary to remove or otherwise reduce the corresponding oxide layer. In one embodiment, the amount of hydrocarbon injected during a single oxide-dissolving event may be about 100-1000 ppm or about 1/100-1/1000 of a total amount of fuel consumed (i.e., including fuel used for normal combustion purposes) by power system 10 during the event, and have an injection duration of about 5-300 seconds. This amount of hydrocarbon may serve primarily to remove some or all of the oxide layer and have little affect on the air-to-fuel ratio or the temperature of exhaust within passage 26. For example, the injected hydrocarbon may raise the air-to-fuel ratio of the exhaust within passage 26 by less than about 5% and increase a temperature of the exhaust by less than about 30° C. In one example, the injection of hydrocarbon has been shown to remove the oxide layer to less than about 20% of the substrate surface of DOC 44 in as little as about 5-300 seconds, depending on the application.

In one embodiment, controller 52 may delay the injections of hydrocarbon until the exhaust temperatures have peaked and cooling of the exhaust is observed. This cooling may correspond, for example, with idling of power system 10 or a particular segment of an excavation cycle such as a dump or return segment that requires less output from power system 10. Controller 52 may determine that temperatures have peaked and the exhaust is cooling when controller 52 detects a temperature drop of about 5-20° C. over a one minute time period.

Under some conditions, the exhaust temperatures of power system 10 may remain elevated for extended periods of time. During operation in these conditions, controller 52, after triggering the initial hydrocarbon injections according to the strategy outlined above, may continue to inject bursts of hydrocarbon as long as exhaust temperatures remain elevated. For example, after the initial burst of hydrocarbon, injector 46 may be controlled to inject subsequent bursts of hydrocarbon about every five minutes.

INDUSTRIAL APPLICABILITY

The exhaust system of the present disclosure may be applicable to any power system having an oxidation catalyst, where continued performance at a desired level is important. The performance of DOC 44 may be extended through selective injections of hydrocarbon into the exhaust flow of power system 10 at a location upstream of DOC 44 when it is determined to be likely that an oxide layer has formed on DOC 44. Operation of power system 10 will now be described.

Referring to FIG. 1, air induction system 16 may pressurize and force air or a mixture of air and fuel into combustion chambers 14 of power system 10 for subsequent combustion. The fuel and air mixture may be combusted by power system 10 to produce a mechanical work output and an exhaust flow of hot gases. The exhaust flow may contain a complex mixture of air pollutants composed of gaseous material, which can include oxides of nitrogen (NO_(X)). As this NO_(X)-laden exhaust flow is directed from combustion chambers 14 through oxidation catalyst 44, some NO in the flow may be converted to NO₂.

After passing through oxidation catalyst 44, the exhaust flow containing an increased amount of NO₂ may be directed through DPF 32 and SCR 34. As the exhaust passes through these treatment devices, particulate matter in the exhaust may be removed by DPF 32 and NO_(X) in the exhaust may be reduced to innocuous substances. As described above, the increased amount of NO₂ generated by DOC 44 may facilitate passive regeneration of DPF 32 and enhance the reduction of NO_(X) within SCR 34.

When temperatures of the exhaust flow passing through DOC 44 reach about 180° C. and/or remain elevated above 180° C. for at least 2 sec., an oxide layer may form on the metallic substrate of DOC 44. This oxide layer, if unaccounted for, may decrease the performance of DOC 44. To maintain the desired level of performance within DOC 44, the substrate of DOC 44 may need to be selectively regenerated. Accordingly, controller 52 may monitor the signals from exhaust sensor 54 (Step 300) and determine if exhaust temperatures have remained above 180° C. for at least 2 seconds (Step 310). When controller 52 determines that the conditions of step 310 have been satisfied (Step 310: Yes), controller 52 may await the next exhaust cooling event (e.g., an event where exhaust temperatures cool by about 5-20° C. within a one minute time period) (Step 310), and then initiate regeneration of DOC 44 by causing injector 46 to selectively inject bursts of hydrocarbon (i.e., diesel fuel) into the exhaust of passage 26 at a location upstream of DOC 44 (Step 330). This injected hydrocarbon, when it comes into contact with the oxide layer, may facilitate a chemical reaction that removes the oxide layer and restores functionality to DOC 44. As long as controller 52 determines that exhaust temperatures remain elevated (e.g., above 180° C.), controller 52 may cause injector 46 to inject additional bursts of hydrocarbon on a regular basis, for example every five minutes (Step 340), without waiting for a cooling event to occur.

Several aspects may be associated with power system 10. For example, the disclosed injections of hydrocarbon may be capable of dissolving the oxide layer of a DOC in a very short amount of time with very little hydrocarbon. In particular, it has been shown that a relatively small injection of hydrocarbon (i.e., about 1/100-1/1000 of a total amount of fuel consumed) may dissolve the oxide layer in about 5-300 seconds, and do so with an efficiency of about twenty times greater than using elevated concentrations of NO alone. Accordingly, the disclosed system may be very responsive and efficient. In addition, many power systems may already be equipped with an exhaust-located fuel injector used to actively regenerate a DPF and/or heat an SCR device. Accordingly, selective use of this same fuel injector to regenerate a DOC may require little or no new hardware.

It will be apparent to those skilled in the art that various modifications and variations can be made to the system of the present disclosure without departing from the scope of the disclosure. Other embodiments will be apparent to those skilled in the art from consideration of the specification and practice of the system disclosed herein. For example, in addition to utilizing injector 46 to increase an amount of hydrocarbon passing through DOC 44 after formation of the oxide layer, it is contemplated that the air/fuel ratio adjusting devices discussed above (e.g, the throttle valve, the variable valve actuators, the VGT, etc.) may also be utilized to help in removing the oxide layer. Further, it is contemplated that, instead of utilizing injector 46 to inject fuel and selectively remove the oxide layer from DOC 44, fuel injectors 15 may additionally be utilized to inject the small quantities of fuel required for the removal at a timing when the injected fuel will not fully combust within cylinders 14 (e.g., in a late post injection or during an injection when corresponding cylinders 14 are disabled). When fuel injectors 15 are utilized to regenerate DOC 44, injector 46, supply 48, and device 50 may be omitted. It is intended that the specification and examples be considered as exemplary only, with a true scope of the disclosure being indicated by the following claims and their equivalents. 

1. An exhaust system for a combustion engine, comprising: an exhaust passage configured to receive a flow of exhaust from the combustion engine; an oxidation catalyst disposed within the exhaust passage; a fuel injector configured to selectively inject fuel into the exhaust at a location upstream of the oxidation catalyst; a temperature sensor configured to generate a signal indicative of a temperature of exhaust flowing through the exhaust passage; and a controller in communication with the fuel injector and the temperature sensor, the controller being configured to: make a determination based on the signal that an oxide layer has formed on the oxidation catalyst; and regulate operation of the fuel injector to inject fuel and reduce the oxide layer based on the determination.
 2. The exhaust system of claim 1, wherein the controller is configured to make the determination that an oxide layer has formed when the signal indicates a temperature of exhaust flowing through the exhaust passage has exceeded a threshold temperature.
 3. The exhaust system of claim 2, wherein the controller is configured to make the determination that an oxide layer has formed when the signal indicates the temperature of exhaust flowing through the exhaust passage has remained above the threshold temperature for a threshold period of time.
 4. The exhaust system of claim 2, wherein the threshold temperature is a temperature at which about 60-100% of the oxidation catalyst is covered by the oxide layer.
 5. The exhaust system of claim 4, wherein the fuel injected by the fuel injector to reduce the oxide layer changes an air-to-fuel ratio of the flow of exhaust by less than about 5%.
 6. The exhaust system of claim 4, wherein the fuel injected by the fuel injector to reduce the oxide layer raises temperatures by less than about 30° C.
 7. The exhaust system of claim 4, wherein the fuel injected by the fuel injector during a single burst to reduce the oxide layer is about 100 to 1000 ppm in the flow of exhaust, and the controller is configured to continue the injections at about five minute intervals as long as the temperature of exhaust flowing through the exhaust passage remains above the threshold temperature.
 8. The exhaust system of claim 2, wherein the controller is configured to regulate operation of the fuel injector to inject fuel when the signal indicates the exhaust flowing through the exhaust passage is cooling.
 9. The exhaust system of claim 1, further including an exhaust treatment device disposed within the exhaust passage downstream of the oxidation catalyst.
 10. The exhaust system of claim 9, wherein the exhaust treatment device is one of a particulate filter configured to regenerate in the presence of NO₂ generated by the oxidation catalyst or a reduction device configured to reduce a constituent of the exhaust in the presence of NO and NO₂ generated by the oxidation catalyst.
 11. A method of operating an exhaust system, comprising: directing exhaust through an oxidation catalyst; making a determination that an oxide layer has formed on the oxidation catalyst; and selectively introducing a burst of fuel into exhaust directed through the oxidation catalyst to reduce the oxide layer based on the determination.
 12. The method of claim 11, wherein making the determination includes determining that a temperature of the exhaust has exceeded a threshold temperature.
 13. The method of claim 12, wherein making the determination further includes determining that the temperature of the exhaust has remained above the threshold temperature for a threshold period of time.
 14. The method of claim 12, wherein the threshold temperature is a temperature at which about 60-100% of the oxidation catalyst is covered with the oxide layer.
 15. The method of claim 14, wherein selectively introducing bursts of fuel increases an air-to-fuel ratio of the exhaust by less than about 5%.
 16. The method of claim 14, wherein selectively introducing bursts of fuel increases a temperature of the exhaust by less than about 30° C.
 17. The method of claim 14, wherein the fuel injected during a single burst of fuel is about 100 to 1000 ppm in the exhaust, and the method further includes continuing to introduce bursts of fuel at about five minute intervals as long as the temperature of the exhaust remains above the threshold temperature.
 18. The method of claim 11, wherein selectively introducing bursts of fuel includes selectively introducing bursts of fuel when the exhaust is cooling.
 19. The method of claim 11, further including directing exhaust from the oxidation catalyst through at least one of a particulate filter and a reduction catalyst.
 20. A power system, comprising: an internal combustion engine configured to combust fuel and generate a flow of exhaust; an exhaust passage leading from the internal combustion engine to the atmosphere; an oxidation catalyst disposed within the exhaust passage to convert NO to NO₂; a particulate filter disposed downstream of the oxidation catalyst configured to passively regenerate in the presence of the NO₂; a fuel injector configured to selectively inject fuel into the exhaust passage at a location upstream of the oxidation catalyst; a temperature sensor configured to generate a signal indicative of a temperature of exhaust flowing through the exhaust passage; and a controller in communication with the fuel injector and the temperature sensor, the controller being configured to: make a determination based on the signal that temperature indicative of formation of an oxide layer on the oxidation catalyst has been exceeded; and regulate operation of the fuel injector to inject fuel and reduce the oxide layer based on the determination, wherein the fuel injected by the fuel injector changes an air-to-fuel ratio of the exhaust by less than 5% and raises a temperature of the exhaust by less than 30° C. 